FISHES OF COLORADO 1 23 



able changes in the volume of water to be carried. During a warm 

 spring day large quantities of snow may be melted on rocky slopes 

 where loss of water by absorption is slight. Thus there is a flooding 

 of canyons and gulches. 



The combined action of all of these factors results in limiting the 

 fish fauna of the mountain streams to a few well-adapted species. 

 Fishes to live in such streams must be hardy, able to endure con- 

 stantly cold water. They must be strong swimmers capable of moving 

 against the strong current of these streams. Indirectly, but effec- 

 tively, the nature of the mountain streams prohibits certain types of 

 fishes, since the feeding-grounds required by these species are not 

 possible under mountain stream conditions. The rapidly moving 

 mountain stream has little or no fringe of aquatic shore vegetation 

 (see Fig. 55), since such plants as might gain a foothold during the low 

 water of the summer season would be swept away by the fall and 

 spring rises. This removes a very important feeding-ground for 

 many species, since the zone of vegetation near shore shelters snails, 

 insect larvae and small Crustacea, forms which taken collectively 

 comprise the main food of sunfishes, top-minnows and many minnows. 

 The strong current in itself and by the removal of silt and water- 

 logged material from the bottom of the stream also limits the possible 

 food for fishes in the stream proper to small snails, caddis-fly larvae 

 and neuropterous larvae of various sorts, and the green and brown 

 slime, algae and diatoms, on the stones. 



The census of the fish fauna of these mountain streams shows it 

 to consist of the two Dace, Rhinichthys cataractae dulcis and Agosia 

 yarrowi; of suckers of the genus Pantosteus and of Catostomus com- 

 mersonii sucklii which feed upon the algal and diatomaceous slime, 

 insect larvae and small snails; of the sculpin, Cottus punctulatus , 

 which feeds upon insect larvae, snails and small fishes; and of the 

 several species of trout which feed upon small fishes, surface insects 

 and larvae of the larger aquatic insects. In addition to these species 

 the Darters, Boleosoma nigrum mesaeum and Etheostoma iowae, are 

 found in the lower portions of the mountain streams in the foothill 

 region east of the Divide in the Platte drainage. Besides these there 



